Process for cleaning harmful gas

Abstract: There is disclosed a process for cleaning a harmful gas which comprises bringing a gas containing an alkoxide compound such as tetraethoxysilane and trimethylphosphorous acid or a silane compound such as monosilane and disilane as a harmful component into contact with a cleaning agent comprising an alkali metal compound such as potassium hydroxide, sodium hydroxide and potassium oxide supported on a metallic oxide comprising cupric oxide and manganese dioxide as principal components. According to the above process, it is made possible to effectively and safely remove the harmful component in the gas exhausted from semiconductor manufacturing process in an emergency or in ordinary case irrespective of its concentration.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for cleaning a harmful gas. More particularly, it pertains to a process for cleaning a harmful gas containing any of various alkoxide compounds and silane compounds that are used in the production step of liquid crystal or CVD step in semiconductor manufacturing process. In detail, the present invention relates to a process for cleaning a harmful gas which comprises bringing a gas containing a harmful component into contact with a cleaning agent which comprises an alkali metal compound supported on a carrier to remove said harmful component from said harmful gas. 2. Description of the Related Art With the continuous development of semiconductor industry accompanied by high degree of integration and miniaturization of the related products, there has been a steady rise in recent years in the amounts of alkoxide compounds typified by tetraethoxysilane and silane compounds typified by monosilane. In addition, pentaethoxytantalum which is one of alkoxide compounds has brought into use in CVD step as a raw material for tantalum oxide film which is employed for a semiconductor device from a new compound or the like, for example, an interlaminar insulating film. Most of the above-mentioned alkoxide compounds and silane compounds are highly toxic or unknown in toxicity and are combustible. These alkoxide compounds and silane compounds have made themselves indispensable as raw materials or doping gases in the production process for silicon-based semiconductor and liquid crystal. At any rate it is necessary to remove a harmful component in the gas after being used in the semiconductor production process prior to discharge in the atmosphere. As a process for cleaning a gaseous alkoxide compound, gaseous silane compound or the like contained in a gas, there are available two kinds of processes, one being wet process in which harmful components are absorbed in an absorbing solution composed principally of aqueous solution of an acidic component by the use of a scrubber; and the other being dry process which make use of activated carbon, a porous adsorbent of inorganic compound type, metalic salt supported on activated carbon, etc. (Japanese Patent Application Laid-Open No.207426/1991) However, the above-mentioned wet process suffers the disadvantages that when the equipment used therefor is large-sized, it finds difficulty in post-treatment, it requires a considerable expense in the maintenance of the equipment and further that the absorption efficiency is not necessarily high, thereby making it impossible to immediately and completely treat a highly concentrated harmful gas at the present time. In addition the wet process brings about the problem of sludge formation due to the generation of a powdery oxide which is produced by the decomposition of tetraethoxysilane, monosilane or the like. The dry process exemplified by the above-mentioned adsorptive treatment with activated carbon is disadvantageous in its low removing capability of harmful components and further requires a countermeasure against the possible hazard of causing fire when there is used a combustible or inflammable gas such as monosilane, and at the same time, a combustible gas such as hydrogen or a combustion-supporting gas such as oxygen each as a diluting or carrier gas. A process as detailed in the chapter "Field of the Invention" is known from the document FR-A-2609905. Therein is disclosed the use of soda lime, NaOH and potassium permanganate supported on pumice as well as the use of NaOH and Cu-Mn-Co oxide supported on pumice. The solid adsorbents serve to adsorb silicon hydrides. A process known from the document EP-A-0495392 comprises oxidation of waste gases from CVD processes at the surface of oxidation catalysts selected from the group consisting of MnO 2 , Cu, Pt and Pd. Such being the case, there has long been desired the materialization of a process for cleaning a harmful gas, especially that discharged from semiconductor industry, said process being enhanced in both treatment rate and treatment capacity for harmful gas, excellent in removal performance not only in an emergency when a relatively less concentrated but a large amount of a harmful gas is leaked from a gas cylinder because of its abnormality but also in the ordinary case where a harmful gas is discharged in high concentration after being used in semiconductor production process, and free from the possibility of causing the hazard of fire in the case of gas cleaning or the clogging in a cleaning column due to sludge formation. SUMMARY OF THE INVENTION Under such circumstances, intensive research and investigation were concentrated by the present inventors in order to develop a process for efficiently cleaning a harmful gas such as an alkoxide compound by overcoming the above-mentioned problems. As a result it has been found that the use of a cleaning agent comprising an alkali metal compound supported on a metallic oxide which comprises cupric oxide and manganese dioxide as principal components is capable of removing the above mentioned alkoxide compound in high efficiency and in safety. The present invention has been accomplished on the basis of the aforesaid finding. Specifically, the present invention provides a process for cleaning a harmful gas which comprises bringing a gas containing as a harmful component at least one alkoxide compound represented by the chemical formula M(OR) X , wherein M is a metallic element, OR is an alkoxy group and X is the valency of the metal, into contact with a cleaning agent which comprises an alkali metal compound selected from the group consisting of "hydroxide, oxide or carbonate of potassium or sodium" or mixtures of compounds selected from said group, supported on a metallic oxide comprising cupric oxide and manganese dioxide, wherein the total amount of the cupric oxide and manganese oxide is at least 60% by weight based on the total amount of the metallic oxide, to remove said harmful component from said harmful gas. The process for cleaning a harmful gas according to the present invention is applicable to a harmful gas of an alkoxide compound-basis represented by the chemical formula given in claim 1, which compound is exemplified by tetraethoxysilane, triethoxyarsenic, triethoxyboron, trimethylphosphoric acid, triethylphosphoric acid, trimethylphosphorous acid, triethylphosphorous acid, pentaethoxytantalum and tetraisopropoxytitanium. In particular, the process according to the present invention is capable of rapidly cleaning a large amount of harmful gas at ordinary temperature and thus exerts excellent effect on the cleaning of the gas exhausted from semiconductor manufacturing process and rapid cleaning of the air polluted with a harmful gas due to sudden leakage from a gas cylinder. There is employed in the present invention, a cleaning agent comprising an alkali metal compound supported on a metallic oxide. The metallic oxide comprises cupric oxide and manganese dioxide as principal components and may be incorporated with cobalt (III) oxide, silver (I) oxide, silver (II) oxide, aluminum (III) oxide, silicon dioxide or the like. The content of the total sum of cupric oxide and manganese dioxide in the composition comprising said two components as the principal ones is at least 60 % or more, preferably 70 % or more by weight. The ratio manganese dioxide to cupric oxide (CuO:MnO 2 ) is usually about 1:0.8 to 5, preferably 1:1.2 to 3.0 by weight. The metallic oxide-based composition comprising the above-mentioned cupric oxide and manganese dioxide as principal components can be prepared by a publicly known method such as mixing of each component but are conveniently available on the market under the trademark Hopkalite, which is mainly a binary composition comprising 40 % by weight of cupric oxide and 60 % by weight of manganese dioxide. It sometimes further contains an oxide of potassium, magnesium, aluminum, silicon or the like in an amount of about 30 % or less by weight in addition to the aforesaid two components. At any rate, the Hopkalite may be employed as such or in the form of mixture with at least one metallic oxide as described hereinbefore. The metallic oxide to be used in the present invention is usually in the form of molding and may be in the form of crushed product, extruded molding, tableted molding or the like. The preferable size thereof is about 4 to 20 mesh for the crushed product, about 1.5 to 4 mm in diameter by 3 to 20 mm in height for the extruded molding and about 3 to 6 mm in diameter by 3 to 6 mm in height for the tableted molding which is usually cylindrical. Examples of the alkali metal compound to be supported on the metallic oxide include a hydroxide, an oxide, a carbonate each of potassium, sodium or lithium, and such compound is specifically exemplified by potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium oxide, sodium oxide, potassium carbonate, sodium carbonate and a mixture of at least two of them. Of these are preferable potassium hydroxide, sodium hydroxide, potassium oxide and potassium carbonate. The amount of the alkali metal compound to be supported on the metallic oxide varies depending on the kind, concentration, etc. of the alkoxide compound or the silane compound contained in the gas to be treated and thus can not be unequivocally determined. It is however, usually 1 to 50, preferably 3 to 15 parts by weight expressed in terms of each alkali metal hydroxide based on 100 parts by weight of the metallic oxide. An amount thereof less than 1 part by weight results in lowered cleaning efficiency, whereas that more than 50 parts by weight leads to both difficulty in being sufficiently supported on the metallic oxide and expensiveness of its own. It is desirable in the present invention that the cleaning agent in addition to the alkali metal compound be made to preserve free water to some extent for the purpose of enhancing the efficiency of removing harmful components. The amount of free water to be preserved is usually 1 to 40 %, preferably 5 to 30 % by weight based on the total amount of the cleaning agent. The cleaning agent can be prepared by, for example, dissolving an alkali metal compound in water, impregnating a metallic oxide with the resultant aqueous solution and then drying the impregnated metallic oxide at about 30 to 100 °C to attain a prescribed water content. The concentration of a harmful gas to which the present invention is applicable is usually 1 % or lower, preferably 0.1 % or lower. The temperature of contact between the cleaning agent and the harmful gas is about 0 to 100 °C, but it is usually room temperature (10 to 50 °C), thus dispensing with heating and cooling. In addition, temperature rise in the range of 10 to 40 °C due to reaction heat is sometimes observed after the start of contact in the case where the harmful gas has a high concentration or contains a highly reductive gas such as hydrogen. In no case, however, is caused abnormal heat generation. The pressure at the time of contact between the cleaning agent and the harmful gas is usually atmospheric pressure, but the cleaning operation can be put into practice at a reduced pressure or under pressure such as 1 kg/cm 2 G. Air, nitrogen or hydrogen which contains a harmful gas and to which the process according to the present invention is applied may be either in dry state or wet state to the extent that moisture condensation is not caused. In most cases, the harmful gas is cleaned at a humidity corresponding to the relative humidity of the atmosphere ranging from 30 to 100 %. In such a case, the free water content in the cleaning agent is preferably about 5 to 30 % by weight. In the cleaning process according to the present invention, there is no specific limitation to the concentration of the alkoxide-based component in the gas to be treated and the flow velocity of the gas. In general however, the flow velocity is desirably lowered with increasing concentration. The concentration at which the harmful components is easily removed by the contact with the cleaning agent is usually 1 % or less by volume, but can be increased in the case of lower flow rate of the harmful gas. The cleaning column is designed in accordance with the concentration of harmful gas, the amount of the gas to be treated, etc. The space linear velocity (LV) is preferably designed at 0.5 to 50 cm/sec for a relatively low concentration of harmful gas such as 0.1 % or lower by volume, 0.05 to 20 cm/sec for a concentration of 0.1 to 1 % by volume and 10 cm/sec or less for a high concentration such as 1 % or higher by volume. Thus the general design criteria specify a LV value of 10 cm/sec or less in the case where a concentrated harmful gas is exhausted in a steady state from semiconductor production process, and a LV value of 0.5 to 50 cm/sec in the case where a harmful gas is suddenly leaked from a gas cylinder and diluted with a large amount of air or the like. The cleaning agent is usually packed in a cleaning column, which is generally in the form of fixed bed and may be in the form of mobile bed or fluidized bed. The alkoxide-based compounds as harmful components contained in the gas are removed by passing the gas through the cleaning column to bring the gas into contact with the cleaning agent. The bulk density of the cleaning agent is about 1.0 to 1.5 g/ml when it is packed in a cleaning column. According to the process for cleaning harmful gas of the present invention it is made possible to effectively remove alkoxide compound such as tetraethoxysilane that are contained in a gas irrespective of concentration ranging from low to high concentration. Furthermore even when a strongly reductive gas such as hydrogen is present, the harmful components can rapidly and safely be removed without causing abnormal heat generation. Thus the process of the present invention can exhibit excellent effect when applied to the cleaning of exhaust gas from semiconductor manufacturing process or an emergency harm-removal equipment for the gas leaked from a gas cylinder. In the following, the present invention will be described in more detail with reference to examples, which however shall not be construed to limit the present invention thereto. Example 1 to 4 (Preparation of cleaning agent) Potassium hydroxide in amounts of 5 to 250 g was dissolved in 400 to 800 ml of water in a vat, and each of the solutions was incorporated with 500 g of Hopkalite (CuO : 40 % by weight, MnO 2 : 60 % by weight, produced by Nissan Girdler Co., Ltd.), followed by drying at 100 °C to prepare four 4 kinds of cleaning agents each having a different content of potassium hydroxide and different free water content as given in Table 1. By the use of each of the cleaning agents thus prepared, cleaning test was carried out for a harmful gas by a method wherein dry nitrogen containing 1711 ppm by volume of tetraethoxysilane was allowed to flow through a cleaning column made of quartz glass with 19 mm inside diameter and 500 mm length which had been packed with 85 ml of any of the above-prepared cleaning agents, at 20 °C under atmospheric pressure at a flow rate of 510 ml/min (equivalent to space linear velocity (LV) of 3.0 cm/sec). Then, the treated gas at the outlet of the cleaning column was sampled, and the sample gas was analyzed by means of gas chromatograph (produced by Shimadzu Corporation, model GC-12A) to determine the period of time required for tetraethoxysilane contained in the treated gas to reach the upper limit of the permissible concentration (10 ppm) in terms of effective treatment time (min). The results are given in Table 1. Example 5 to 7 The procedure in Example 1 was repeated to carry out gas cleaning test except that the cleaning agent same as that in Example 3 was employed and alteration was made to the concentration of tetraethoxysilane and the flow-throught velocity (space linear velocity (LV)) of the gas. The results are give also in Table 1. Example 8 to 9 The procedure in Example 1 was repeated to carry out gas cleaning test except that the cleaning agent same as that in Example 3 was employed; trimethylphosphorous acid in a content of 7894 ppm was used as harmful component in place of 1711 ppm tetraethoxysilane; alteration was made to flow-through velocity (LV); and measurement was made of the period of time required for trimethylphosphorous acid contained in the treated gas to reach 10 ppm. The results are given also in Table 1.

Claims: A process for cleaning a harmful gas which comprises bringing a gas containing as a harmful component at least one alkoxide compound represented by the chemical formula M(OR) X , wherein M is a metallic element, OR is an alkoxy group and X is the valency of the metal, into contact with a cleaning agent which comprises an alkali metal compound selected from the group consisting of "hydroxide, oxide or carbonate of potassium or sodium" or mixtures of compounds selected from said group, supported on a metallic oxide comprising cupric oxide and manganese dioxide, wherein the total amount of the cupric oxide and manganese oxide is at least 60% by weight based on the total amount of the metallic oxide, to remove said harmful component from said harmful gas. The process according to Claim 1 wherein said metal compound is selected from the group consisting of potassium hydroxide, sodium hydroxide, potassium oxide and potassium carbonate. The process according claim 1 wherein the amount of said alkali metal compound supported on said metallic oxide is 1 to 50 parts by weight expressed in terms of alkali metal hydroxide based on 100 parts by weight of said metallic oxide. The process according to claim 1, wherein said alkoxide compound as a harmful component is at least one compound selected fron the group consisting of tetraethylorthosilicate, triethoxyarsenic, trimethylphosphorous acid, triethylphosphorous acid, trimethylphosphoric acid, triethylphosphoric acid and triethoxyboron. The process according to claim 1 wherein said cleaning agent preserves 1 to 40% by weight of free water. The process according to claim 1 wherein said harmful gas is brought into contact with said cleaning agent at a temperature of 100 °C at the highest.

Inventors: HATAKEYAMA TOSHIYA, OKUMURA TOSHIO, SHIMADA TAKASHI, HATAKEYAMA, TOSHIYA, OKUMURA, TOSHIO, SHIMADA, TAKASHI, HATAKEYAMA, TOSHIYA, C/O JAPAN PIONICS CO., LTD., OKUMURA, TOSHIO, C/O JAPAN PIONICS CO., LTD., SHIMADA, TAKASHI, C/O JAPAN PIONICS CO., LTD.
Applicants: JAPAN PIONICS, JAPAN PIONICS CO., LTD.
Application year: 1994
Main classification: B01D53
IPC codes: B01J20, B01D53